A critical moment.
17... f6f7 A good move, preventing the threat of e4-e5+. 18. a1a4 Threatening the bishop on b4. 18... c4c3  19. d1d3 A dangerous rook lift, remembering Black that the king still isn't entirely safe. 19... a7a5 Reinforcing the position of the bishop on b4, but accuracy is demanded by Black.